Device for generating a strong electronic beam from a plasma emitting cathode



Feb. 13, 1968 c. JACOBSEN ETAL 3,369,142

DEVICE FOR GENERATING A STRONG ELECTRONIC BEAM FROM A PLASMA EMITTING CATHODE Filed April 11, 1966 10 M l a m J m ,3

7 a A--, A

INVENTORS CLHS uncossan r30 BRE/THOLTZ 197 7 ORNEXS United States Patent 3,369,142 DEVICE FOR GENERATING A STRONG ELEC- TRON IC BEAM FROM A PLASMA EMITTING CATHODE Clas Jacobsen and Bo Breitholtz, Vasteras, Sweden, as-

signors to Allmiinna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Filed Apr. 11, 1966, Ser. No. 541,678 Claims priority, application Sweden, Apr. 12, 1%5, 4,719/ 65 2 Claims. (Cl. 313-170) ABSTRACT OF THE DISCLOSURE A device for generating a strong electronic beam has a vacuum vessel; a cathode is arranged in the vessel. An anode means comprises an annular acceleration anode and a collector anode. The anode means may be arranged either inside or outside the vessel. The cathode is surrounded by insulating material except for that surface turned away from the anode means. An auxiliary anode is arranged in the vicinity of the cathode to start a plasma-emitting cathode spot on the uninsulated cathode surface. The cathode and the auxiliary anode are provided with condensation screens of insulating material.

In electronic valves and similar apparatus a heated cathode is usually used as the electronic source. Heated cathodes have however relatively low current density and are therefore not suitable when high current density is desired in the electronic beam.

In order to obtain an electronic beam with high current density, a mercury cathode may be used as the electronic source. Ions and neutral gas atoms are also emitted from the cathode spot on such a cathode, which ions and atoms are thrown out in straight paths in all directions away from the cathode. The ions however return relatively quickly to the cathode, but the neutral atoms which are not influenced by the field in the arc continue and at a relatively great distance from the cathode may be ionised by electrons, which means that ions may occur practically anywhere between the anode and the cathode.

In many cases it is desirable to have an electronic beam which is completely free from ions and neutral atoms and the present invention relates to a device for generating such a strong and pure electronic beam between a cathode arranged in a vacuum and an anode. The invention is characterised in that the cathode is surrounded by insulating material except on a surface turned from the anode and that the plasma which expands from a cathode spot generated on said surface acts as the electronic source in the device.

The pure and high energetic electronic beam obtained can be used for many different purposes, such as welding, melting of metals in vacuum in order to obtain as far as possible gas-free metals and alloys, etc.

The invention will be described below with reference to the accompanying drawing which shows schematically a section through an embodiment of the device according to the invention.

A cathode 2, a catch anode 3, an acceleration anode 4 and an auxiliary anode 5 are arranged in a vacuum container 1. The cathode is surrounded by an insulating layer 6 of for example ceramic material. Only the upper surface 7 is free from the coating and on this surface is situated the ion and electron emitting cathode spot 8. The auxiliary anode 5 is arranged immediately above the surface 7 "ice of the cathode and is raisable and lowerable through a wall of the container 1. The cathode and the auxiliary anode are connected to an auxiliary voltage source 9. A screen 10 is inserted between the cathode and the auxiliary anode in order to prevent a current in the auxiliary circuit.

On the auxiliary anode a condensation screen 11 is fixed by means of an insulating intermediate layer 12 and also the cahtode is provided with such a condensation screen which in the figure is denoted by 13 and is fixed at the insulating layer 6 surrounding the cathode. The purpose of the condensation screens is to catch the neutral atoms which are emitted from the cathode spot and to prevent them from being transferred to the walls of the container.

The acceleration anode 4 is placed between the cathode and the catch anode. The field in the acceleration anode is to be shaped so that the greatest possible part of the electronic current passes straight through the anode and reaches the catch anode. In order to focus the electronic beam and in order to prevent its electrostatic dispersion, a longitudinal magnetic field can be connected in.

The discharge is started by lowering the auxiliary anode until it makes contact with the cathode surface 7 and then raising it so that an arc occurs between the two electrodes. When the arc leaves the cathode surface the cathode spot 8 appears. Electrons, ions and neutral metal atoms are emitted from the cathode spot and these form a plasma over the cathode. The ions and the neutral metal gas atoms can only move in straight paths and this is the reason why the plasma only exists above the straight line AA in the figure. The electrons on the other hand can be influenced by the field from the main voltage source 14 and the potential of these can be chosen to be high if it is desirable to have a high are voltage drop and obtain a beam of high-energy electrons against the catch anode 3. The electronic beam becomes very strong, since a cathode spot is used as the electronic source and the area of the cathode spot is to a great extent unlimited. What sets an upper limit for the electronic current is that at cur-rent strengths of several tens of kA the properties of the plasma above the cathode are determined by collision between the particles of the plasma, and neutral metal atoms are spread to the area below the line AA.

Instead of using a movable auxiliary anode for igniting the are, a high ohmic control electrode can be arranged on the cathode surface. A third possibility is to start the discharge by means of a focus laser beam directed against the cathode.

If the conditions are such that the plasma generating cathode spot is stable without the aid of the are between the auxiliary anode and the cathode, the current source 9 can be disconnected as soon as the cathode spot has been created and the discharge stabilised.

The cathode must be manufactured of an extremely gas-free material in order that the quantity of neutral gas which is released during the fusing of the cathode will be as small as possible. The cathode material can be solid or liquid metal, such as copper or mercury.

The catch anode can be arranged outside the container and the electronic beam can be taken out through a window arranged in the wall of the container between the acceleration anode and the catch anode. rIt is also possible to arrange both the acceleration anode and the catch anode outside the vacuum container.

We claim:

1. A device for generating a strong electronic beam comprising anode means and a cathode, at least said cathode being arranged in a vacuum vessel, insulating material covering said cathode laterally and on the side facing said anode means, the surface of said cathode turned away from said anode means being uninsulated, whereby plasma which expands from a cathode spot generated on said surface constitutes an electronic source in the device, an auxiliary anode means in the vicinity of said cathode for starting a plasma emission, and a condensation screen of insulating material between said cathode and said auxiliary anode means.

2. A device according to claim 1, said auxiliary anode means and said cathode being provided with metallic 0ondensation screens, said condensation screens being insulated from said auxiliary anode means and said cathode.

References Cited UNITED STATES PATENTS 1,929,124 10/1933 Smith 313l89 X 2,686,275 8/1954 Cohen 313-74 2,805,365 9/1957 Mulder 313-489 X 2,937,300 5/1960 Hernquist 3 l3l89 JAMES W. LAWRENCE, Primary Examiner.

0 DAVID J. GALVIN, Examiner.

V. LAFRANCHI, Assistant Examiner. 

